Bicycling has enjoyed a great revival of interest in recent years. This has been due in part to recent emphasis on exercise as well as environmental considerations such as the pollution problems caused by extensive automobile use. With bicycles being used in a variety of riding conditions, multi-speed bicycles, and particularly 10-speed bicycles, have come into widespread use. These bicycles include means for varying the transmission ratio as between the pedaling force and the rear wheel of the bicycle, the ratio being periodically adjusted by the rider in accordance with the speed obtained and the grade of the terrain. The conventional transmission ratio changing means on five and 10-speed bicycles is the so-called "derailleur" mechanism. Derailleur mechanisms utilize multiple side by side sprockets of different diameters and operator-controlled shift levers are utilized to activate the derailleur mechanism which, in turn, causes motion of the chain in a direction lateral to the sprocket axes. In this manner, the chain is caused to move from one sprocket to another of greater or lesser diameter.
There are a number of well-recognized problems with derailleur type systems, although satisfactory improvements or substitutes therefor have not been previously attained. The chain jumping from sprocket to sprocket can readily "hang up" or, at best, be subjected to wear which necessitates realignment and/or replacement of parts. In certain gears there is a substantial lateral displacement of the chain as between its front and rear sprockets, and this makes pedalling difficult and inefficient as well as causing greater than average wear. This serves to render some of the gears of a 10-speed bicycle virtually unusable. All present chain-drive bicycles are quite susceptible to the problems caused by road dirt, and the amount of friction and stress encountered by a derailleur mechanism during shifting makes this mechanism particularly susceptible to problems caused by road dirt.
A further disadvantage of derailleur mechanisms is the difficulties they pose from the standpoint of maintainability. For example, disassembly of at least a part of the mechanism and chain is necessary when removing the rear wheel. Also, under present designs, the chain is "trapped" around the lower support bar of the bicycle, so that it is necessary to "break" the chain at one link in order to remove or replace it.
There have been a number of recent attempts to device alternatives to the conventional derailleur, some of the proposed techniques being described in an article entitled "New Transmissions: More Push-Power for Your Bike" which appeared in the May 1974 issue of POPULAR SCIENCE. These types of systems are also described in an article entitled "Product Report" at page 43 of the February 1974 issue of BICYCLING and in U.S. Pat. Nos. 3,798,989, 3,800,613, and 3,850,044. In one of these systems a mechanism changes the primary gearing ratio by increasing or decreasing the distances between six miniature sprockets which are coupled together on hinged arms and move radially in unison to vary the effective diameter of the "sprocket group." Each sprocket is provided with a clutch shoe which is frictionally engageable with a common clutch plate on a drive shaft. In one system, the miniature sprockets ride in and out in the grooves of a spiral "scroll" plate. A disadvantage of these types of systems is that since all the miniature sprockets are coupled together it is difficult for them to move outward under load, so the rider is required to pedal backwards in order to shift up. Also, these types of systems appear to be limited in their application to the front sprocket assembly since their design is not suitable for rear wheel sprockets due to size limitations. This tends to limit the gear ratio range capabilities of such devices.
In another of the systems described in the above-referenced publications and patents, a set of sliding gear segments are radially interposed under operator control. This system has only a limited number of available gear ratios, however. Also, all of the prior art problems attendant the use of an exposed gearing mechanism and chain are still present.
It is an object of this invention to provide a bicycle drive transmission system which overcomes remaining problems in the prior art, as set forth.